1. Field of the Invention
The invention relates to a nickel-chromium-iron-aluminum alloy having excellent high-temperature corrosion resistance, good creep resistance, and improved processability.
2. Description of the Related Art
Austenitic nickel-chromium-iron-aluminum alloys having different nickel, chromium, and aluminum contents have been used in furnace construction and in the chemical process industry for a long time. For this use, good high-temperature corrosion resistance and good heat resistance/creep resistance even at temperatures above 1000° C. is required.
In general, it should be noted that the high-temperature corrosion resistance of the alloys indicated in Table 1 increases with an increasing chromium content. All of these alloys form a chromium oxide layer (Cr2O3) with an Al2O3 layer that lies underneath and is more or less closed. Slight additions of strongly oxygen-affine elements such as Y or Ce, for example, improve the oxidation resistance. The chromium content is slowly consumed during the course of use in the region of application, to build up the protective layer. For this reason, the useful lifetime of the material is increased by means of a higher chromium content, because a higher content of chromium, as the element that forms the protective layer, delays the point in time at which the Cr content is below the critical limit and oxides other than Cr2O3 form, which are oxides that contain iron or that contain nickel, for example. A further increase in the high-temperature corrosion resistance can be achieved by means of addition of aluminum and silicon. Starting from a certain minimum content, these elements form a closed layer below the chromium oxide layer, and thereby reduce the consumption of chromium.
The heat resistance/creep resistance at the temperatures indicated is improved by means of a higher carbon content, among other things.
Examples of these alloys are listed in Table 1.
Alloys such as N06025, N06693 or N06603 are known for their excellent corrosion resistance in comparison with N06600, N06601 or N06690, because of the high aluminum content. Alloys such as N06025 or N06603 also demonstrate excellent heat resistance/creep resistance even at temperatures above 1000° C., because of the high carbon content. However, the processability, e.g. formability and weldability, are impaired by these high aluminum content values, whereby the impairment is all the greater, the higher the aluminum content (N06693). The same holds true to an increased degree for silicon, which forms intermetallic phases with nickel that melt at a low temperature. For N06025, for example, it was possible to achieve weldability by means of the use of a special welding gas (Ar with 2% nitrogen) (data sheet for Nicrofer 6025 HT, ThyssenKrupp VDM). The high carbon content in N06025 and N06603 results in a high content of primary carbides, which leads to crack formation, proceeding from the primary carbides, for example at high degrees of forming, as they occur during deep drawing, for example. Something similar happens during the production of seamless pipes. Here, too, the problem becomes worse with an increasing carbon content, particularly in the case of N06025.
EP 0 508 058 A1 discloses an austenitic nickel-chromium-iron alloy consisting of (in weight-%) C 0.12-0.3%, Cr 23-30%, Fe 8-11%, Al 1.8-2.4%, Y 0.01-0.15%, Ti 0.01-1.0%, Nb 0.01-1.0%, Zr 0.01-0.2%, Mg 0.001-0.015%, Ca 0.001-0.01%, N max. 0.03%, Si max. 0.5%, Mn max. 0.25%, P max. 0.02%, S max. 0.01%, Ni remainder, including unavoidable melting-related contaminants.
EP 0 549 286 discloses a high-temperature-resistant Ni—Cr alloy containing 55-65% Ni, 19-25% Cr, 1-4.5% Al, 0.045-0.3% Y, 0.15-1% Ti, 0.005-0.5% C, 0.1-1.5% Si, 0-1% Mn, and at least 0.005% in total of at least one of the elements of the group that contains Mg, Ca, Ce, <0.5% in total Mg+Ca, <1% Ce, 0.0001-0.1% B, 0-0.5% Zr, 0.0001-0.2% N, 0-10% Co, remainder iron and contaminants.
From DE 600 04 737 T2, a heat-resistant nickel-based alloy has become known, containing ≦0.1% C, 0.01-2% Si, ≦2% Mn, ≦0.005% S, 10-25% Cr, 2.1-<4.5% Al, ≦0.055% N, in total 0.001-1% of at least one of the elements B, Zr, Hf, whereby the stated elements can be present in the following contents: B≦0.03%, Zr≦0.2%, Hf≦0.8%. Mo 0.01-15%, W 0.01-9%, whereby a total content Mo+W of 2.5-15% can exist, Ti 0-3%, Mg 0-0.01%, Ca 0-0.01%, Fe 0-10%, Nb 0-1%, V 0-1%, Y 0-0.1%, La 0-0.1%, Ce 0-0.01%, Nd 0-0.1%, Cu 0-5%, Co 0-5%, remainder nickel. For Mo and W, the following formula must be fulfilled:2.5≦Mo+W≦15  (1)